1. Field of the Invention
This invention relates to a separator for a fuel cell and a fuel cell therewith.
2. Description of the Related Art
Recently, there has been substantial interest in a fuel cell which shows a higher energy conversion efficiency and generates no harmful materials from a power-generating reaction. A known example of such a fuel cell is a polymer electrolyte fuel cell which can operate at a low temperature of 100° C. or lower.
A polymer electrolyte fuel cell is an apparatus having a basic structure in which a solid polymer membrane as an electrolyte membrane is disposed between a fuel electrode and an air electrode, and generates electric power while feeding a fuel gas containing hydrogen to the fuel electrode and an oxidizer gas containing oxygen to the air electrode, according to the following electrochemical reactions.Fuel electrode:H2→2H++2e−  (1)Air electrode:1/2O2+2H++2e−→H2O  (2)
In the polymer electrolyte fuel cell, hydrogen ions can be conducted when the solid polymer electrolyte membrane is in a wet condition. The fuel and the oxidizer gases (hereinafter, sometimes referred to as “reaction gas” or “reaction fluid”) are, therefore, generally fed after being humidified.
In a fuel electrode, hydrogen contained in a fuel fed is decomposed into hydrogen ions and electrons as shown in equation (1). The hydrogen ions move toward the air electrode through the solid polymer electrolyte membrane while the electrons move to the air electrode via an external circuit. On the other hand, in the air electrode, oxygen contained in an oxidizer fed to the air electrode reacts with the hydrogen ions and the electrons from the fuel electrode to form water as shown in equation (2). Thus, in the external circuit, electrons move from the fuel electrode to the air electrode so that electric power can be collected.
Outside of the fuel electrode and the air electrode, separators are provided. An anode is disposed on one surface of the solid polymer electrolyte membrane while a cathode is disposed on the other surface to form a membrane electrode assembly (MEA). The membrane electrode assembly is sandwiched between the gas separators to provide a cell unit. A plurality of the cell units are laminated and clamped as an integral part using a penetrating rod in a lamination direction, to form a cell stack. The separator in the side of the fuel electrode has a fuel gas channel for feeding a fuel gas to the fuel electrode. Similarly, the separator in the side of the air electrode has an oxidizer gas channel for feeding an oxidizer to the air electrode. Furthermore, between these separators, there is a cooling water channel for cooling the cell unit.
As described above, insufficient wetting of the solid polymer electrode membrane may cause inadequate electrochemical reaction, leading to deteriorated power-generation performance. Thus, in the prior art, a reaction gas is humidified and then fed to a gas separator such that the humidified reaction gas flowing in the channel keeps the solid polymer electrode membrane wet.
For making a fuel cell popular for household applications, it must be small and light as well as exhibit improved output properties and safety. Thus, there have been also investigated a separator for a fuel cell for making it smaller and lighter and for excellent properties (see, for example, Japanese Laid-open Patent Publication Nos. 2001-43868 and 2000-294261).
The separator described in Japanese Laid-open Patent Publication No. 2001-43868 does not have a configuration suitable for discharging water generated by the reaction represented by equation (2) and condensation of the wet reaction gas in the fuel cell. Water deposited in the channel for the reaction gas inhibits smooth flowing of the reaction gas. It may lead to insufficient feeding of the reaction gas to an electrode and resultantly unstable operation of the fuel cell. The technique described in Japanese Laid-open Patent Publication No. 2000-294261 is not satisfactory for achieving a compact fuel cell exhibiting improved output properties. In particular, a reaction gas is generally humidified by a humidifier before introduction into the system, and the gas is cooled in a manifold for feeding the reaction gas, generating a large amount of condensed water. When the condensed water is deposited on an inlet for a reaction gas in a separator or enters a reaction gas channel from the inlet for a reaction gas, the reaction gas channel is obstructed. It may inhibit even feeding of the reaction gas to the electrode surface, leading to reduction in a fuel cell output.
The phenomenon will be separately described in the sides of a cathode and an anode. When a wet reaction gas flows in the channel in the gas separator, water generates by an electrode reaction in the cathode side, so that as an oxidizer gas flows in the channel downstream, a water vapor concentration increases, leading to formation of condensed water, which is deposited on the channel. On the other hand, in the anode side, hydrogen is consumed by an electrode reaction, resulting in reduction of the volume of the fuel gas and increase in a water vapor concentration due to back-diffusion of water from the cathode side, and finally as in the cathode side, condensed water is deposited on the channel. In the channel with condensed-water deposition, a channel resistance for the reaction gas is higher than a channel without water deposition, distribution in a reaction gas flow rate may be observed among a plurality of channels in the gas separator, and/or sometimes distribution in a reaction gas flow rate may be observed among cell units constituting a cell stack, compared with a channel without water deposition. In such a case, feeding of the reaction gas may be insufficient, leading to deteriorated power-generation properties in a channel or cell unit to which a flow rate of the reaction gas is inadequate.
To solve the above problems, the number, the size (cross-sectional area) or the pattern of channels are varied to prevent condensed water from being deposited on the channel in the prior art. For example, there has been disclosed a technique that a width and a depth of a channel groove are selected to preventing retention of condensed water in the channel (Japanese Laid-open Patent Publication No. 1994-96777). Alternatively, Japanese Laid-open Patent Publication Nos. 2000-149966 and 2001-307753 have disclosed a conventional technique for draining water.
The solid polymer membrane itself is strongly acidic, the atmosphere in an operating fuel cell is strongly acidic. During long-term operation of the fuel cell under such strongly acidic conditions, corrosion resistance is sometimes inadequate even when a gas separator is coated with silver having a smaller ionization tendency. When the surface of the gas separator is eroded, ions of constituting metals in the gas separator are eluted. Thus, when metal ions (silver ions or metal ions constituting a substrate in a gas separator coated with silver) are eluted from the gas separator and then the ions penetrate into a polymer electrode membrane even in a trace amount, the metal ions are attracted to an ion-exchange group (sulfonic group) in the electrolyte membrane, leading to loss of proton conductivity in the solid polymer electrode membrane, which is undesirable for maintaining fuel cell performance. There has been, therefore, needed a gas separator for a fuel cell exhibiting improved corrosion resistance.
Furthermore, in a conventional fuel cell, a separator constituting a unit cell is characterized in that there are provided a plurality of continuous grooves to be channels for a reaction gas in which the periphery is flat and the central part is different between the front and the rear faces; the ends of the grooves are inclined; the peripheral flat area is sealed by a seal member; and the separator is made of stainless steel or titanium, and gas channels are provided within a substantially square area in the separator (see, for example, Japanese Laid-open Patent Publication No. 2002-25586, page 1, FIG. 1).
As illustrated in FIG. 24 (cited from FIG. 2 in WO 00/39872), there has been disclosed a fuel cell with a cylindrical protrusion at the center of a flat region wherein around the protrusion, a plurality of parallel cell operation parts 320a comprising a substantially square separator equipped with an inlet and an outlet for a fuel gas, an inlet and an outlet for an oxidizer gas and an inlet and an outlet for cooling water are tightly fastened as an assembly, via a pair of a right and a left collector plates 321a, 321b made of a conductive material such as stainless steel and a pair of a right and a left supporting plates 322 made of an insulating synthetic resin such as polypropylene, with a plurality of fixing bolts (see, for example, WO 00/39872, pp. 9-13, FIG. 2).
Related Art List
Japanese Laid-open Patent Publication No. 2001-43868,
Japanese Laid-open Patent Publication No. 2000-294261,
Japanese Laid-open Patent Publication No. 1994-96777,
Japanese Laid-open Patent Publication No. 2000-149966,
Japanese Laid-open Patent Publication No. 2001-307753,
Japanese Laid-open Patent Publication No. 2002-25586, and
WO 00/39872
A conventional separator for a fuel cell is not satisfactory for achieving a small and light fuel cell exhibiting improved output properties.
In the conventional separator constituting the cell for a fuel cell described in Japanese Laid-open Patent Publication No. 2002-25586, gas channels are provided within the substantially square region. Thus, the number of the gas channels are increased and a substantial amount of gas fed is divided into many channels so that a flow rate for each channel is reduced. As a result, a reaction efficiency of the gas per a given time period is reduced and further, water generated in an air electrode is deposited in the channels. Finally, gas flow is obstructed, leading to reduction in a power-generation efficiency.
In the fuel cell described in WO 00/39872, a cell operation part comprises substantially square-shaped separators as a component, so that the cell operation part is also a rectangular parallelepiped. Therefore, the plurality of fixing bolts for fastening and collector plates for collecting electricity generated from the cell operation part are placed outside of the cell operation part. Thus, a fuel cell requires the cell operation part for generating electric power, but the fuel cell becomes larger than the cell operation part due to the fixing bolts and the collector plates.
Furthermore, it is not necessarily adequate to prevent deposition of condensed water in the channel in the gas separator according to Japanese Laid-open Patent Publication No. 1994-96777. In particular, many droplets are generated in the downstream of the channel because water generated by an electrochemical reaction enters the channel. As shown in FIG. 31, in a gas separator 401 in which at the outlets of channels 402, there is provided a concave header 403 at which the plurality of channels 402 are joined, water droplets generated in the downstream of the channels 402 is gradually discharged by the reaction gas and deposited, resulting in a large amount of retained water 404 in the header 403. A large amount of the retained water 404 in the header 403 inhibits discharge of the reaction gas (unreacted reaction gas) to a manifold 405 following the header 403. As a result, the reaction gas cannot smoothly flow in the channel 402, and further, distribution of the reaction gas becomes uneven, leading to deteriorated power-generation performance.
In view of the circumstances, an objective of this invention is to provide a separator for a fuel cell where a fuel, oxidizer and/or coolant are evenly and effectively fed. Another objective of this invention is to a separator for a fuel cell where condensed water in a fuel or air can be effectively removed from a unit cell in the fuel cell. Another objective of this invention is to provide a separator for a fuel cell with good corrosion resistance. Another objective of this invention is to provide a separator for a fuel cell exhibiting excellent output properties and stability. Another objective of this invention is to provide a fuel cell and a separator for a fuel cell where condensed water derived from a reaction fluid can be effectively discharged from a fuel cell. Another objective of this invention is to provide a fuel cell and a separator for a fuel cell whereby a higher output can be stably obtained.
Another objective of this invention is to provide a fuel cell and a separator for a fuel cell which are small and light and have an improved power-generation efficiency.
Another objective of this invention is to provide a polymer electrolyte fuel cell adapted to prevent a large amount of retained water from generating within a header provided at the outlet of the separator.